Colloquium Toward a new synthesis: Major evolutionary trends in the angiosperm fossil record David Dilcher* Florida Museum of Natural History, University of Florida, Gainesville, FL 32611-7800 Angiosperm paleobotany has widened its horizons, incorporated Nature and Origin of Primitive Angiosperms,’’ there is no new techniques, developed new databases, and accepted new substantive use of the fossil record to address this question. questions that can now focus on the evolution of the group. The The theories and hypothesis presented by Stebbins are based fossil record of early flowering plants is now playing an active role on the comparative morphology and anatomy of living angio- in addressing questions of angiosperm phylogeny, angiosperm sperms considered primitive at that time rather than the fossil origins, and angiosperm radiations. Three basic nodes of angio- record of early angiosperms. sperm radiations are identified: (i) the closed carpel and showy However, at the same time, the early 1970s, special attention radially symmetrical flower, (ii) the bilateral flower, and (iii) fleshy was being focused on the fine features of the morphology of fruits and nutritious nuts and seeds. These are all coevolutionary angiosperm leaf venation and the cuticular anatomy of living and events and spread out through time during angiosperm evolution. fossil angiosperms (2, 6–8). Most of the early angiosperms from The proposal is made that the genetics of the angiosperms pres- the Cretaceous and early Tertiary were being found to be extinct sured the evolution of the group toward reproductive systems that or only distantly related to living genera (Fig. 3). Grades and favored outcrossing. This resulted in the strongest selection in the clades of relationships were being founded on the basis of careful angiosperms being directed toward the flower, fruits, and seeds. character analysis (9, 10). During this time, it became scientif- That is why these organs often provide the best systematic char- ically acceptable to be unable to identify a modern genus to acters for the group. match a fossil. Fossil angiosperms were analyzed on the basis of multiple detailed objective characters, and degrees of relation- ere I focus on the fossil record of the same plants that ships could be established based on the extent to which these HStebbins did in his book, Variation and Evolution of Plants same combinations of characters were found in living families, (1), the angiosperms. This contribution has the advantage of subfamilies, or genera (11). Analyses of the fossil angiosperm being written more than 50 years after Stebbins wrote about his record were being constructed that included vast amounts of view of the fossil record of the angiosperms. His use of the fossil data based on careful anatomical and morphological analysis of record of angiosperms as a model in his evolutionary synthesis the diversity of characters found in living genera and modern was hampered because in the 1940s the paradigm in angiosperm families. Large collections of cleared leaves and cuticular prep- paleobotany was to match fossils, especially leaves, to extant arations were developed, and whole families were surveyed to genera (2). The successes of the angiosperm paleobotanists (e.g., establish their range of venation and cuticular characters and D. Axelrod, H. Becker, E. W. Berry, R. Brown, R. Chaney, and fruit and seed anatomy and morphology to research the fossil H. MacGinitie, and many others for 100 years before them) were history of a family (4, 9, 11–24). This anatomical͞morphological judged by their ability to match a high percentage of fossils to style of systematic-based angiosperm paleobotany was a distinct living genera (Fig. 1). Once the identifications were made to change from the floristic approaches that focused on paleogeo- living genera, their focus was on questions of phytogeography graphic and paleoclimatic questions and dominated the field and paleoclimate. This meant that almost no fossil angiosperms before 1970. were recognized as extinct; it was quite impossible to focus This new paradigm shift opened the door for a new synthesis questions of plant evolution on the fossil record of the angio- of the fossil record of angiosperms. New questions about the sperms in 1950 as George Gaylord Simpson had done with the evolutionary biology of the fossil record of angiosperms could fossil vertebrate record in his classic Tempo and Mode in now be addressed based on detailed character-based data of Evolution in 1944 (3). living and fossil angiosperms often organized with the help of Stebbins wrote in ‘‘Fossils, Modern Distribution Patterns cladistic analysis. At this same time there was renewed interest and Rates of Evolution,’’ chapter 14 of Variation and Evolution in exploring the fossil plant record to determine the origin and of Plants (1), about the disjunct distribution of modern genera early evolutionary history of the angiosperms (8, 25, 26). The of fossil plants. His rates of evolution were based on the various techniques of careful analysis and the concerted effort to open modern genera described in the fossil record of North America up a new fossil record of early angiosperms by the use of small, and currently living in southeastern Asia or South America. often charcoalified plant remains (27), or fragments of cuticle His arguments about the rates of evolution from the fossil sieved from sediment (28) from newly collected material of the record may have some validity when based on fossils from the Jurassic to the Upper Cretaceous were very successful. A Miocene (about 25 million years) and younger. However, many whole new area of the study of intermediate-sized fossil plants, of the fossils from the Paleocene, Eocene, and Oligocene often termed mesofossils as opposed to microfossils or reported as living genera have been subject to revisions (4) as megafossils, expanded to occupy the majority of angiosperm shown in Fig. 2. This trend that had dominated angiosperm paleobotany for more than 100 years continued into the early 1970s. The supposed failure of the fossil record to contribute This paper was presented at the National Academy of Sciences colloquium ‘‘Variation and to understanding the evolution of the early angiosperms was Evolution in Plants and Microorganisms: Toward a New Synthesis 50 Years After Stebbins,’’ still evident in 1974 when Stebbins published Flowering Plants: held January 27–29, 2000, at the Arnold and Mabel Beckman Center in Irvine, CA. Evolution Above the Species Level (5). In chapter 10, ‘‘The *E-mail: dilcher@flmnh.ufl.edu. 7030–7036 ͉ PNAS ͉ June 20, 2000 ͉ vol. 97 ͉ no. 13 Downloaded by guest on September 29, 2021 Fig. 1. Selected floras published from the late 1800s to the 1960s, ranging in time from the Lower Cretaceous to the Upper Eocene (62–69). The open area represents percent of the species in the flora that were given extant generic names. The shaded area represents the percent of species in the flora that were given fossil generic names based on a modern genus to which they were perceived to be similar. The short fall, less than 100% for each flora, represent genera perceived to be truly extinct. research in some laboratories with good success (refs. 29–36 fossil record. Much of this analysis is based on the study of the and references cited therein). It is the success of these new anatomy and morphology of fossil plant organs. Stebbins (1) techniques applied to the fossil record of angiosperms that recognized the need for this type of study when he wrote that now provides a new database from which to analyze some of ‘‘The method of identification is simple comparison between the COLLOQUIUM the major trends in angiosperm evolution and allows us to ask fossil and the leaves of living species, but various approaches new questions. have greatly increased its accuracy.’’ He then cites Bandulska (37), Edwards (38), and Florin (39), all early pioneers in the use What Is Known About Early Angiosperm Diversity During The of anatomy and morphology in the study of the systematics of Cretaceous? fossil plants. However, using the new tools, it became apparent New Tools of Analysis. The study of angiosperm fossils has un- that there were many fossils that could not be related to living dergone rapid and profound changes during the past 30 years as taxa even when such careful analyses were applied (Figs. 2 and discussed in the introduction to this paper. Although the study 3). There came a time when it was necessary to give names to the of angiosperm fossils is only as reliable as the individual inves- various organs of fossil angiosperms that reflected their extinct tigator, resources are now available, such as cleared leaf collec- nature, recognizing them as separate from any living genus (40). tions and cuticular reference slide collections from vast herbar- With few exceptions (41), workers have not yet taken such bold ium holdings. This allows angiosperm paleobotanists to survey steps as defining and naming extinct angiosperm plant families, the nature of characters circumscribed by a particular living orders, or classes. family or genus before reaching a conclusion about the relation- ship(s) of a fossil angiosperm organ. It is now understood that Rapid Changes in the Data. The great strides in developing tech- some organs may contain more useful characters for determin- niques of investigation for understanding Devonian fossil plants, ing relationships than others. It is not only acceptable, but on the basis of seemingly nondescript structurally compressed desirable, to list the available characters and how these are remains (42, 43), and the excellent application of anatomy and distributed among several living genera in a family rather than morphology to the study of Pennsylvanian age plants (i.e., ref. 44, to select only a single living genus that has such characters and and examples cited in ref.